Mutations in a gene called cystic fibrosis transmembrane regulator (CFTR) cause CF. More than 900 mutations in this gene have been found. This gene provides the instructions for the CFTR protein. In normal cells, the CFTR protein acts as a channel that allows cells to release chloride and other ions. But in people with CF, this protein is defective and the cells do not release the chloride. The result is an improper salt balance in the cells which leads to thick, sticky mucus. When mucus clogs the lungs, it can make breathing very difficult, and causes bacteria to get stuck in the airways, resulting in inflammation and infections. Over time, mucus buildup and infections can lead to permanent lung damage, including the formation of scar tissue (fibrosis) and cysts in the lungs. Mucus can also block the digestive tract and pancreas, leading to digestive problems.
 
Inheritance
 CF is inherited in an autosomal recessive manner. This means that to have CF, a person must have a mutation in both copies of the CFTR gene in each cell. People with CF inherit one mutated copy of the gene from each parent, who is referred to as a carrier. Carriers of an autosomal recessive condition typically do not have any signs or symptoms.

When two carriers of an autosomal recessive condition have children, each child has a:
25% (1 in 4) chance to have CF
50% (1 in 2) chance to be a carrier of CF like each parent
25% chance to not have CF and not be a carrier

When a carrier of CF has a child with a person with CF, each child has a:
50% (1 in 2) chance to have CF
50% (1 in 2) chance to be a carrier of CF